Response.Redirect

Hi..
Am using response.redirect() to go to another page from the current page. But it is giving me the exception "System.PlatformNotSupported",Operation not supported, change the pipeline mode of IIS.
Am working in VS 2008,IIS v5.1

Sorry, cant share the code.
It's like I have a funtcion in the page_load() and I call that function in the very beginning of page _load().
In that function,I check for some conditions and if true,am calling the Response.Redirect() to another page.
But the control is going back to the page_load() of the same page.

I put it in the debug mode to see the control flow.
Control is coming to the page_load() , from there it is going to other function,there I have my condition. It's nothing related to the session thing. I have a grid view and am checking which item got clicked in that function. Even that also is working and then control is going to the response.redirect line. From there control again is going back to the page_load() of the same page. Before response.redirect line,am setting one session variable.